Hollow-core doors are commonly used in many types of buildings. The common structure of a hollow-core door includes a pair of door skins that are connected at their perimeters by a rectangular frame, which holds the skins apart to form a hollow space. The skins are commonly made of medium-density fiberboard (MDF), which is an engineered wood product made by combining wood fibers with a binder, and applying high temperature and pressure to mold the fibers into a desired shape. The skins may be formed into completely flat, planar surfaces that are parallel to each other, forming what is known as a flush door, which gives the door a uniform thickness and constant internal width. Alternatively each skin may be formed into a contoured surface with one or more molded panels recessed into the exterior surface of the door, which creates raised panels and panel ridges on the hollow inside of the door. As used herein, parallel skins means that a plane of the bottom skin is parallel to a plane of the top skin, even though the skins may be comprised of multiple planes due to the raised panels. The width of the hollow space between the skins varies across the length and width of the door, as it is reduced by the depth of the raised panel on each skin. This gives the door a non-uniform thickness on the outside and between the skins. A lock block may also be included in the area of the door where locks and handles are attached to provide the additional support that is needed to secure a lock in the door.
As known in the prior art, hollow-core doors are assembled lying flat on a horizontal surface. A bottom door skin is positioned on the horizontal surface with its inside surface 7 facing up. See FIG. 1. Then one or more spacers is attached to the bottom skin and the top skin is placed on top of the spacer(s), forming the door with the hollow interior. The orientations of “top” and “bottom” referenced herein relate to doors and door skins lying horizontally, as opposed to the upright position a door is in when it is opened and closed within a door jamb and lintel.
Hollow-core doors are less structurally sound than solid doors, and more prone to twist and bend. In addition, some hollow-core door skins are so thin that, over the length of a door, the skins tend to sag toward each other. To give the door structural rigidity and prevent the skins from falling into the hollow core, a support structure is placed in the hollow core between the skins and adhered to them. The support structures are usually made of corrugated fiberboard. Corrugated fiberboard is made of a fluted sheet of fiberboard adhered between two flat sheets of fiberboard. Corrugated fiberboard is very resistant to being crushed in a direction parallel to the lengthwise axis of the flutes.
Several types of door support structures are known in the art. For example, U.S. Pat. No. 5,875,608 discloses an expandable spacer that is made of rigid elongated members connected at intersections to foldable connecting members. The elongated members can be collapsed parallel together at the intersections by folding the connecting members, similar to an accordion. When expanded, the elongated and connecting members are at right angles to each other to form a grid or honeycomb pattern. Enough elongated and connecting members are used that the resultant grid fills nearly the entire hollow core. This type of support structure does not provide uniform support within a paneled door, because the spacer rests on the edges and mesas of the raised panels, leaving sizeable gaps between the spacer and the skins where there are no raised panels. Because the grid fills nearly the entire hollow core, the support uses a lot of cardboard, which adds cost and weight.
U.S. Pat. No. 5,875,609 discloses another expandable spacer that accommodates the raised panels by cutting notches in the elongated members where they will cross the raised portions of the panel. The connecting members are not notched. U.S. Pat. No. 5,992,127 discloses another expandable spacer in which both the elongated members and connecting members are notched to accommodate the raised panel portions. Again, enough elongated members and connecting members are used that the resultant grid fills nearly the entire hollow core, resulting in high cost and weight. Unfortunately, these notched expandable spacers are difficult to install because the honeycomb does not stretch evenly, and the notches permit the members to twist and bend at the thinned area, so the spacers are difficult to align where desired.
Such expandable spacers are supplied initially in an unexpanded form to save space during transport and storage. To install between skins, glue is applied to the unexpanded form of the spacer and it is then stretched across one skin of the interior of the door. The second skin is placed on the open glue-covered top surface of the spacer to form the door. One problem with using such an expandable spacer is that it is difficult to stretch in a way that achieves an even grid pattern within the door. Commonly, the spacer must be overstretched and then manipulated into place. This process is labor intensive and thus not a cost effective manner for manufacturing the door. In addition, these expandable spacers fully extend between the horizontal rails and vertical stiles to completely fill the hollow interior. Consequently, more core material is inserted within the interior of the door than is actually required for support. The extra core material increases the cost of the door.
U.S. Pat. No. 6,132,836 discloses a spacer that is not expandable, and is formed by gluing together layers of corrugated fiberboard or expanded or extruded polystyrene foam to form a stack, then cutting the stack perpendicular to the longitudinal axis of the flutes. This results in rigid blocks of corrugated cardboard or polystyrene that can nonetheless be broken into the desired length by hand without scoring. Changing the number of layers changes the thickness of the spacer, so that some blocks are thick enough to support the skins apart at locations without panels, and some are thick enough to support the skins apart at locations with panels. The blocks are glued to the inside of the skins at strategic locations. Each block requires a lot of glue, and because the blocks are not notched, no single block can span a raised portion of the panel. See FIGS. 1 and 2. Another disadvantage of solid blocks spacers is that they too use a large quantity of material, making them more expensive.
FIGS. 1, 2, and 3 show prior art. In one prior art example, a rigid block support 9 is made by stacking corrugated sheets of cardboard on top of each other, gluing the flat surfaces together, and cutting the glued stacks into long rectangular blocks. The rigid block support 9 is initially made in long pieces and then broken into shorter pieces to fit into the desired areas between the skins, such as between raised panels. To make it easier to manually break the long pieces into shorter pieces, the long pieces are perforated along perforation lines 3. See FIG. 2. The perforations reduce the strength of the blocks at the perforations.
To build the door, the rigid block supports 9 are manually broken into appropriately-sized pieces and glue is applied to the fluted edges of the rigid block supports that will rest on the bottom skin 7. The bottom glue-covered portions are placed on the inside surface of a bottom door skin 7 between raised panels 8, with the open ends of the flutes against the skin. See FIG. 1. More glue is applied to the fluted edges of the rigid block supports that the top skin 17 will rest on. The top skin 17 is placed on to on top of the glue-covered spacers, forming the door with the hollow interior.
The rigid block support 9 of the prior art has straight edges 4 along its lengthwise (y) axis. For this reason the rigid block supports 9 are not placed across the perimeter of the raised panels 8 because to do so would cause the straight edge to rest at an angle on the ridge of the panels, effectively raising one end of the rigid block support 9 off the skin. Rigid block supports 9 are not placed directly on the raised panels 8 either, because since the top skin 17 rests on the rigid block supports 9, the width of the hollow interior space would be greatly increased and would leave the hollow areas between the panels completely unsupported. In other words, rigid block supports 9 cannot simultaneously be adhered to both the base and raised portions of the door skins. Because the rigid block supports 9 cannot traverse the raised panels, many rigid block supports 9 pieces are needed to fully support the bottom and top skins apart from each other, requiring time and manual labor for braking the shorter pieces the desired length and placing them all.
In another prior art example, as shown in FIG. 3, the spacer has first and second elongated members 1. A plurality of rigid crossmembers 2 are coupled to and extend between the first and second elongated members 1 in the z-axis. Each crossmember 2 is oriented perpendicularly to the elongated members 1 and to the door skins 7. This forms a series of compartments 5 open to the skins on their tops and bottoms. To build the door, the compartmented supports are broken into appropriately-sized pieces and glue is applied to the fluted edges that will rest on the bottom skin 7. The glue-covered portions are placed on the inside surface of a bottom door skin 7 between raised panels 8, with the open ends of the flutes against the skin. More glue is applied to the fluted edges of the rigid block supports that the top skin 17 will rest on. The top skin is placed on to on top of the glue-covered compartmented spacers, forming the door with the hollow interior.
The compartmented support has a straight edge along its lengthwise (y) axis and suffers the same problems as with the rigid block supports 9 since it cannot be placed across the perimeter of the raised panels without increasing the width of the hollow interior space. Compartmented supports cannot simultaneously be adhered to both the base and raised portions of the door skins.
Tens of thousands of hollow-core doors are made daily in the US; millions every year. Even small reductions of the amount of material, glue, and labor in the manufacturing process can save millions of dollars. Therefore, it is an object of this invention to provide an internal support for a paneled hollow-core door that uses less paper and glue, has a lower cost of materials and labor, takes less time to assemble, and provides more strength to the door than known supports.